The present invention relates in general to a grounding device, and, more particularly, to a self-testing grounding device having a resistance tester coupled directly to the device.
Static electricity provides problems in a number of industries, most particularly the electronics industries, with the advent of integrated circuits and other microelectronic components. Integrated circuits, for instance, may be disabled or destroyed by over-voltages or high power densities resulting from static electricity. Certain junctions in such circuits can be destroyed by as little as a 50-volt potential, which radically changes the doping structure in their lattices. High power densities resulting from excessive potential and imperfections in circuit layout or structure can vaporize or radically alter the silicon substrate and thus impair or destroy a circuit's performance. Yet a person walking on a carpet on a dry day can accumulate as much as 30,000 volts of potential, and can triboelectrically generate thousands of volts by simply changing position in a chair or handling a styrofoam cup.
A person can inadvertently discharge such static electric potential into a circuit or component by touching it and causing overvoltage or excessive power density. Additionally, the potential in such a person's body can induce a charge in a circuit that can later cause overvoltage or excessive power density when the circuit is subsequently grounded.
More and more frequently, therefore, manufacturers of integrated circuits and other similar microelectronic components are taking measures to limit the failure rate of those circuits and components by attempting to keep them as well as their environment at zero electrical potential. Such measures include providing workers and work stations with antistatic carpet, conductive or dissipative grounded desk top work surfaces, hot air ion generators which emit ions to neutralize static charges, and grounding devices to keep workers at zero potential. The term "conductive" herein, and according to its customary usage in the art, means an electrical resistance of between zero and 10.sup.5 ohms. Similarly, "dissipative" means a resistance of between 10.sup.5 and 10.sup.9 ohms, "antistatic" means a resistance of between 10.sup.9 and 10.sup.14 ohms, and "insulative" means a resistance of more than 10.sup.14 ohms.
A grounding device must have several features in order to perform its grounding function effectively. First, it must ensure that the user's skin is electrically connected to ground. This connection is typically accomplished by a conductive surface on the inside of a strap portion of the device contacting the skin. The conductive surface is electrically connected to a grounding cord which leads from the strap portion to a grounded electrical connection. If the electrical contacting surface on the inside of the strap portion becomes dirty or fouled by oil, perspiration or hair, the strap portion may lose its effectiveness. It is therefore important to form the conductive surface on the inner surface of the strap portion from a conductive material that does not easily become dirty or fouled.
Second, comfort is a premium consideration, because if the strap portion is uncomfortable, the wearer will be tempted to remove it and thereby defeat the grounding function possibly resulting in damage to electrical components on which the wearer is working. A strap portion that is easily stretchable, that breathes, that is attractive and that poses minimum inconvenience to the wearer is therefore highly desired. The situations in which grounding devices or straps are used heightens the importance of their being comfortable so that they are continuously worn and maintain continuous electrical contact with the skin.
Persons working on microelectronic components or integrated circuits may be completely unaware that minor static electrical charges have accumulated, and may therefore unknowingly be in a position to disable circuits. If the strap portion is loose or has been removed, the persons may be unaware that electrical discharges transmitted from their fingers are disabling these circuits. (A typical person cannot sense a static electrical discharge of less than approximately 3,500 volts.) Damage to the circuits may not be discovered until hours, days or weeks later, when the circuits have been placed in components or devices which fail in the field. Removal and repair or replacement of these circuits once in the field is far costlier than avoiding potential failure while the user is handling the circuits. Thus, the user's employer typically must depend upon the effectiveness of the grounding device to maintain a lower failure rate of such electronic circuits and components, by maintaining continuous electrical contact with the user's body.
Third, the proper resistance of the grounding device must be maintained to ensure the user's safety and the effectiveness of the device. The grounding resistance of a typical grounding device is approximately 1 megohm to limit the drain of electrostatic discharge (ESD) and to protect the user in the event of sudden discharge of current. The unrestricted flow of ESD through a "conductive" grounding device could actually damage an integrated circuit while the sudden discharge of current resulting from an electrical short could endanger the safety of the user. Similarly, a grounding device that has too much resistance may not drain charge fast enough which also could result in damage to an integrated circuit.
The user typically tests the resistance of the grounding device one or more times a day at a stand alone resistance tester. The tester is relatively expensive and is usually centrally located so that it may be used by a number of users. Typically, the cord end of the of the grounding device is plugged into the tester while the strap portion is being worn by the user. The tester then determines the resistance of the grounding device and provides an indication whether it is acceptable to continue using the device. Such a tester is inconvenient, and, since each and every user must make a conscious effort to go to the tester to test their grounding devices, its use may be intentionally or inadvertently neglected. Further, the resistance of the grounding device can change during use so that these inconvenient tests preferably are performed multiple times during continuing use of the grounding devices.
Accordingly, there is a need for a grounding device that includes a self-contained resistance tester. Preferably, such a device would be relatively inexpensive, easy to use, and easy to manufacture.